This is a thread I know something about.
D8 vs d6 on a tile plow.
For starters, anything with less than 36" pads forget about it. So that limits you on the d8 until you get into recent tractors.
The d8 hightrack and the d6 lgp have nearly identical length of track on ground, and with 36" pads on both, you will have similar fore-aft stability.
If the soil conditions are good, and traction is ample, the extra weight and horsepower will be a huge benefit in terms of speed and depth.
However, if soil conditions are soft, you cant make use of the power. Both tractors spin out, and you will be pulling both.
If you need to get into swamp holes, you will find a d8 sinks into muck, where the 6 stands a chance of floating. I got that from a guy who had one.
My 6R pulls 12" down to 5' with a lot of pre-ripping in dry clay, and with a 6H winch on the front in wet clay. One way or another, we get it in.
As far as plows go, in the clays we deal with, it seems like the welder is never far away. Perhaps in more loamy soils a Johnson would survive an 8, but not here, no way. Just not enough steel imo. Or give that 8 good traction and hit a rock, you can kiss that johnson goodbye ( figuratively and literally!)
On the other hand, if you can hold that d8 to dry or frozen conditions, it would work great.
Self propelled tile plows are designed to work great in all conditions, and have evolved to big horsepower, long wide cleated tracks providing big flotation, and big weight. The d8 and d6 bring a subset of those qualities. You have to decide which are qualities you need most for the $ you have to spend.
Good luck